Monopole Problem
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In physics, a magnetic monopole is a hypothetical particle that is a magnet with only one pole (see Maxwell's equations for more on magnetic poles). In more technical terms, it would have a net "magnetic charge". Modern interest in the concept stems from particle theories, notably Grand Unified Theories and superstring theories, which predict their existence.
The classical theory of magnetic charge is as old as Maxwell's equations, but is considered much less important or interesting than the quantum theory of magnetic charge, which started with a 1931 paper by Paul Dirac. In this paper, Dirac showed that if magnetic monopoles exist, then that would explain the quantization of electric charge in the universe. Since then, several systematic monopole searches have been performed. Experiments in 1975 and 1982 produced candidate events that were initially interpreted as monopoles, but these are now regarded to be inconclusive. It therefore remains possible that monopoles do not exist at all.
Monopole detection is an open problem in experimental physics. Within theoretical physics, modern approaches agree that monopoles exist. In particular, Grand Unified Theories and string theory both require them. Joseph Polchinski, a prominent string-theorist, described the existence of monopoles as "one of the safest bets that one can make about physics not yet seen". These theories are not necessarily inconsistent with the experimental evidence: in some models magnetic monopoles are unlikely to be observed, because they are too massive to be created in particle accelerators, and too rare in the universe to wander into a particle detector.
Grand Unifying Theories
In more recent years, a new class of theories has also suggested the presence of a magnetic monopole.
In the early 1970s, the successes of quantum field theory and gauge theory in the development of electroweak and the strong nuclear force led many theorists to move on to attempt to combine them in a single theory known as a grand unified theory, or GUT. Several GUTs were proposed, most of which had the curious feature of suggesting the presence of a real magnetic monopole particle. More accurately, GUTs predicted a range of particles known as dyons, of which the most basic state is a monopole. The charge on magnetic monopoles predicted by GUTs is either 1 or 2gD, depending on the theory.
The majority of particles appearing in any quantum field theory are unstable, and decay into other particles in a variety of reactions that have to conserve various values. Stable particles are stable because there are no lighter particles to decay into that still conserve these values. For instance, the electron has a lepton number of 1 and an electric charge of 1, and there are no lighter particles that conserve these values. On the other hand, the muon, essentially a heavy electron, can decay into the electron and is therefore not stable.
The dyons in these same theories are also stable, but for an entirely different reason. The dyons are expected to exist as a side effect of the "freezing out" of the conditions of the early universe, or symmetry breaking. In this model the dyons arise due to the vacuum configuration in a particular area of the universe, according to the original Dirac theory. They remain stable not because of a conservation condition, but because there is no simpler topological state to which they can decay.
The length scale over which this special vacuum configuration exists is called the correlation length of the system. A correlation length cannot be larger than causality would allow, therefore the correlation length for making magnetic monopoles must be at least as big as the horizon size determined by the metric of the expanding universe. According to that logic, there should be at least one magnetic monopole per horizon volume as it was when the symmetry breaking took place. This leads to a direct prediction of the amount of monopoles in the universe today, which is about 1011 times the critical density of our universe. The universe appears to be close to critical density, so monopoles should be fairly common. For this reason, monopoles became a major interest in the 1970s and 80s, along with the other "approachable" prediction of GUTs, proton decay. The apparent problem with monopoles is resolved by cosmic inflation that greatly reduces the expected abundance of magnetic monopoles.
Many of the other particles predicted by these GUTs were beyond the abilities of current experiments to detect. For instance, a wide class of particles known as the X and Y bosons are predicted to mediate the coupling of the electroweak and strong forces, but these particles are extremely heavy and well beyond the capabilities of any reasonable particle accelerator to create.
